It is commonly required to control or regulate the speed of electric motors in equipment such as electric drills, washing machines or vacuum cleaners. The speed should be kept approximately constant, regardless of the mechanical loading of the motor. In such domestic applications, highly accurate motor speed measurement and control is unnecessary.
Known motor speed control circuits fall into two categories. Firstly, they may use open loop control, in that they control the power input to a motor, but have no feedback to determine actual motor speed, and thus are very ineffective, especially when the mechanical load on a motor may change during use. Secondly, they may use a mechanical feedback sensor, such as an optical disk or a magnet, in communication with an optical or magnetic sensor, to measure the speed of rotation of an electric motor. These sensors are expensive, and require a complex installation. The physical alignment of sensors must be accurate, and may require adjustment during installation, which makes the use of speed controlled universal motors costly and complex.
Universal motors powered by an AC voltage may be phase controlled by supplying timed gate pulses to a triac, connected in series with the universal motor. One triac gate pulse per half cycle is supplied, at a certain point in the half cycle. This turns the triac on and supplies current to the universal motor for a portion of each AC half cycle. In response to the measured speed of rotation, the certain point may be adjusted to supply current to the universal motor for a longer or shorter portion of the AC half cycle.
Universal motors may be used indifferently with DC or AC voltages. They have the following electrical characteristics. The instantaneous current i through the universal motor is related to the voltage u across it by the equation: EQU i=u/(Rf+Kv.V) I
where Rf is the resistance of the field winding, V is the speed of rotation of the universal motor, and Kv is a constant depending on the windings and the geometry of the particular universal motor. A universal motor therefore has a resistive characteristic, the equivalent resistance depending on the speed of rotation V of the universal motor. When used with an AC supply voltage, the current flowing in the universal motor becomes out of phase with the applied AC voltage, due to the inductance of the windings of the universal motor.